Optical gratings are useful in controlling the paths of propagating light, particularly light composed of multiple wavelengths. Optical gratings are useful in manipulating the transmittance and/or the propagation direction of particular wavelengths within an optical signal. Since optical signals propagate inside optical waveguides, an optical grating consists of a periodic perturbation (variation) of an optical-waveguide parameter such as the real and/or imaginary part of its refractive index or its thickness. One of the most important types of optical waveguides is the optical fiber. Basically, optical fibers are thin strands of glass capable of transmitting information-containing optical signals over long distances with very low loss. In essence, an optical fiber is a small diameter waveguide comprising a core having a first index of refraction surrounded by a cladding having a second (lower) index of refraction. Provided the refractive index of the core exceeds that of the cladding, a light beam propagated within the core may exhibit total internal reflection, and is guided along the length of the core. Typical optical fibers are made of high purity silica with various concentrations of dopants added to control the index of refraction. Optical fibers that have gratings, perturbations in the refractive index, are of particular interest as components in modern multi-wavelength communication systems, such as wavelength-division-multiplexed optical communication systems.
In-fiber optical gratings are important elements for selectively controlling specific wavelengths of light transmitted within optical systems such as wavelength-division-multiplexed optical communication systems. Such gratings may include short-period fiber Bragg gratings and long-period fiber gratings. These gratings typically comprise a body of material with a plurality of spaced-apart optical grating elements disposed in the material. Often, the grating elements comprise substantially equally-spaced refractive index or optical absorption perturbations. For all types of gratings, it would be highly useful to be able to reconfigure the grating to adjust selectively the controlled wavelengths.
A cladding mode is a mode of light that is not confined to the core, but rather, is confined by the entire waveguide structure. Long-period fiber grating devices selectively forward-diffract light at specific wavelengths by providing coupling between core modes and cladding modes. In general, short-period fiber Bragg gratings can also diffract light into cladding modes. In this case, the cladding modes are back-diffracted. The period, Λ, of the perturbations is chosen to shift transmitted light in the region of a selected peak wavelength, λp, from a core guided mode into a cladding mode, thereby reducing in intensity a band of light having wavelengths centered about the peak wavelength λp. In other words, the fiber grating acts as a band-stop optical spectral filter. In addition, since fiber cladding-modes are weakly-guided modes, their power can be easily dissipated by scattering, bending, stretching, and/or rotating the optical fiber. Such devices are particularly useful for equalizing amplifier gains across a band of wavelengths used in optical communications systems.
Typically, the spacing between the periodic perturbations in a long-period grating is large compared to the freespace wavelength λ of the transmitted light. In contrast with conventional short-period fiber Bragg gratings, long-period gratings use a periodic spacing Λ that is typically about a hundred times larger than the transmitted freespace wavelength. In some applications, such as chirped gratings, the spacing Λ can vary along the length of the grating.
A difficulty with conventional short-period fiber gratings and long-period fiber gratings, however, is their inability to change (tune) dynamically their spectral characteristics. Each short-period fiber grating and each long-period grating with a given periodicity (Λ) selectively filters light with an unchanging attenuation and in an unchanging narrow bandwidth centered around the peak wavelength of coupling, λp. This wavelength is determined by λp=(Ncore±Ncladding) Λ, where Ncore and Ncladding are the guided-mode effective indices of the core and the cladding modes, respectively. The “+” sign is valid for the case of backward-diffracted light by short-period gratings and the “−” sign is valid for forward-diffracted light by long-period gratings. The value of Ncore and Ncladding depend on the wavelength, on the core, cladding, and surrounding medium refractive indices, and on the core and cladding radii.
Various techniques have been developed to extract light from the core of an optical fiber so that the light may be modulated or filtered. In one approach, part of the cladding surrounding the core of the optical fiber is polished away on one side of the fiber so that a portion of the light in the core can be coupled into the cladding. In another approach, disclosed in U.S. Pat. No. 6,058,226, which is hereby incorporated by reference, a voltage is applied to an electrically sensitive material coupled to the exterior an optical fiber. The applied voltage is used for modulating the light being transmitted through the optical fiber. In yet still another approach, disclosed in U.S. Pat. No. 6,055,348, which is hereby incorporated by reference, a longitudinal strain is applied to a fiber grating so that the spacing between the grating elements are changed to shift the wavelength response of the device to provide a tunable optical grating device.
Multi-wavelength communication systems require continuous adjustment of the signal levels. If the signal adjustment is wavelength independent then these devices are called variable optical attenuators (VOA), while for the case of wavelength dependent attenuation they are called variable gain flattening filters. As a first example, in pre-emphasis filtering, some wavelength channels need to be equalized in intensity before they are combined in the fiber. As a second example, the reconfiguration and reallocation of wavelengths among the various nodes of a network by add/drop filtering requires these wavelength channels to be balanced in intensity with the optical network. As a third example, the gain of optical amplifiers, such as erbium-doped optical amplifiers, needs to be the same for all wavelengths, thus requiring wavelength-by-wavelength control of the optical gain. Optical amplifiers have deleterious peaks in their gain spectra that need to be flattened. As a fourth example, an adjustable wavelength and attenuation filter is needed for suppressing amplifier spontaneous emission (ASE) in optical amplifiers. As a fifth example, in a related application, there is a need to control the output power of tunable lasers to be constant over multiple wavelength ranges in order to provide a constant output power over any selected wavelength range.
Multi-wavelength communication systems also require network control functions to be available. As a first example, each wavelength channel should be tagged or labeled. This can be accomplished by modulating each channel wavelength with a slightly different kilohertz frequency. As a second example, network supervisory information needs to be distributed within the existing optical network (without resorting to external wire-based communications) and without affecting any of the data channels within the optical network. This can be done by modulating the existing data channels at kilohertz frequencies with the supervisory information to be distributed.
All of the above needs require a device whose transmission can be controlled in wavelength and amplitude. Adjusting the fiber grating as described in this invention allows tuning of the center wavelength or the adjustment of the attenuation at a fixed wavelength or a combination of these. As such, an adjustable fiber grating is capable of fulfilling all of the above listed application needs. Generally, prior art optical fiber gratings have grating elements that are typically disposed in the optical fiber core and perpendicular to the longitudinal centerline of the optical fiber. However, there are also optical fiber gratings that have grating elements that are slanted, instead of perpendicular, with respect to the centerline of the optical fiber. Several patents also exemplify fiber gratings with slanted refractive-index variation, which are U.S. Pat. No. 5,430,817 to A. M. Vengsarkar, U.S. Pat. No. 5,764,829 to J. Boyd et al. It is accordingly an object of the present invention to provide a new class of fiber gratings.